why not call? he's giving you 6:1. let's take a look at possible hands:

AQ is unlikely (he would have raised preflop)
Q9s unlikely (he would probably have raised the flop)
Q9 unlikely (or he's an idiot)
KJ, K8, J8 makes some sense (he's been slowplaying but many players would have raised earlier)
KT makes some sense (but he should have raised flop)
JT, T8 most probable holdings (then he's been lucky)
AT should have raised preflop (flopped an overcard and a gutshot)
two diamonds makes sense (he just called the flop to get some more people to the turn)
QT and T9 should have raised flop and probably preflop
or he's Bluffing which actually occurs

you beat AT, QT, T9, Two diamonds and a bluff
you lose against AQ, Q9, KJ, K8, KT, J8, JT, T8 and of course slowplayed sets and three T.

you're probably beaten but the odds is still 6:1. and if you lose you may get some interesting info about this guy.